So the argument would be that it wouldn't be able to take much abuse, and possibly injure the wearer more then protect him? but that would of course also apply to leggings ect.
I'm not sure which reasons are involved in "you can have bone legging but not bone armor" or how each reason was weighted. I just looked around to see what I could find about RL bone armor and speculated how one could craft it using "materials available at that time".
This idea is not "possibly injure the wearer more then protect him", but "individual pieces do not disperse force at the point of impact as well as a solid layer of the material does".
Consider this... DF differentiates armor types using the "shaped" state. Armor that is "shaped" is hard and inflexible
(it might bend a bit under mild force). Bone and shell are inflexible materials and can be used for shaped armor, but unlike metals, they cannot be reformed into a larger piece. You can't melt a pile of bones and pour the liquid into a breastplate mold. This might be why these materials are limited to smaller body locations;
where metal armor already consists of a mix of metal plates affixed to a backing material of leather, cloth, or light metal chainmail, then larger pieces of bone or shell could be carved down to replace those metal plates.
Here is an
elephant femur adjacent to little humans. You could cut that bone down to replace the metal pieces used in leggings or greaves, but it is not quite big enough for a breastplate.
Note that shells can be used as a material for "leggings" but not for "greaves", how many natural shells are shaped well for enclosing a dwarf leg? The current code allows any bone to be used, from an elephant or cat, but the bone product from butchering could easily have a size attribute associated with it, allowing bone greaves to be made from only "this size or larger" bones. Also, elves can make wooden breastplates and other armor types, but they tell the tree how to shape it.
Regarding "it wouldn't be able to take much abuse", that is probably handled by the armor damage system. All materials would take damage based on "attacking" material verse "defending" material, and bone is a weaker material than the metals. It would break sooner.
Of course, this is all speculation...